Hot forging steel and method for manufacturing the same

Abstract

The present invention provides a hot forging steel and a method for manufacturing the same, which can suppress grain coarsening even in direct forging and quenching without the normalizing process, and can obtain a high grain size (fine grains). [Solution] Nb-added steel, to which a predetermined amount of Nb has been added to the base steel, can be directly forged and quenched to suppress grain growth by utilizing Nb-based precipitates. This makes it possible to suppress grain coarsening even in direct forging and quenching without the normalizing process, and to ensure a high grain size.

Description

Technical Field

[0001] This invention relates to a steel for hot forging and a method for producing the same.

Background Art

[0002] Recently, while carbon neutrality is being promoted, in the automotive and construction machinery manufacturers, etc., a manufacturing method of direct quenching hot forging products that perform quenching using the waste heat after hot forging has been proposed (for example, see Patent Document 1).

[0003] Since the temperature of the forging at the time of upsetting is at or above the A3 transformation point, reheating (normalizing) can be omitted and quenching can be performed. The omission of this normalizing process leads to cost reduction and reduction of carbon emissions, so it is desirable.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the conventional manufacturing method of direct quenching hot forging products, there is a problem that refinement of crystal grains using reverse transformation cannot be applied.

[0006] That is, when quenching is performed on a product that has been once cooled to room temperature after hot forging, refinement of crystal grains using reverse transformation due to reheating during normalizing is possible. However, when normalizing is omitted, the crystal grains coarsened by preheating before forging or being exposed to high temperatures during forging remain in the forging product as they are, which becomes a factor for deteriorating properties such as impact value.

[0007] This invention is proposed in view of the above-mentioned circumstances, and aims to provide a hot forging steel and a method for manufacturing the same that can suppress grain coarsening even in direct forging and quenching without the normalizing process, and can obtain a high grain size (fine grains). [Means for solving the problem]

[0008] To solve the above-mentioned problems, the hot forging steel according to this application suppresses grain growth by directly forging and quenching Nb-added steel, which is a base steel to which a predetermined amount of Nb has been added.

[0009] The base steel is a boron steel, and each chemical component contains at least 0.35 to 0.39 wt% of C, 0.15 to 0.35 wt% of Si, 0.85 to 1.05 wt% of Mn, 0.90 to 1.00 wt% of Cr, 0.015 to 0.045 wt% of Al, 0.015 to 0.040 wt% of Ti, 0.0005 to 0.0030 wt% of B, and 0.0070 wt% or less of N.

[0010] The boron steel may further contain 0.05 wt% or less of Mo.

[0011] The boron steel may further contain 0.017 wt% of P and 0.021 wt% of S.

[0012] The predetermined amount of Nb is such that 0.02 to 0.10 wt% Nb is added.

[0013] Furthermore, the method for manufacturing hot-forged steel according to this application comprises, as each component, at least 0.35 to 0.39 wt% C, 0.15 to 0.35 wt% Si, 0.85 to 1.05 wt% Mn, 0.90 to 1.00 wt% Cr, 0.015 to 0.045 wt% Al, 0.015 to 0.040 wt% Ti, and 0.0005 to 0.0030 wt% B The process comprises a heating step of applying heat treatment to an Nb-added steel, which is obtained by adding 0.02 to 0.10 wt% of Nb to a base steel containing 0.0070 wt% or less of N, respectively; a forging step of applying hot forging treatment to the Nb-added steel, which is performed continuously with the heating step; and a quenching step of applying hardening treatment to the Nb-added steel using the residual heat from the hot forging treatment. [Effects of the Invention]

[0014] According to this invention, by utilizing niobium (Nb)-based precipitated carbides that are stable within the hot forging temperature range to suppress grain growth during the forging process, it is possible to suppress grain coarsening even in direct forging and quenching without a normalizing process, thereby providing a hot forging steel and a method for manufacturing the same that can obtain a high grain size. [Brief explanation of the drawing]

[0015] [Figure 1] This figure shows a comparison of the chemical composition of forged products produced by direct forging and quenching according to an embodiment of the present invention. [Figure 2] This is a magnified photograph showing an example of the distribution of Nb-based precipitated carbide particles in Nb0.05 steel. [Figure 3] This graph shows the relationship (equilibrium condition) between temperature and the volume fraction of (Nb,Ti)C. [Figure 4] This graph shows the relationship between the amount of Nb added and the grain size in the hot forging temperature range. [Figure 5] This graph shows the relationship between strain rate and grain size, using base steel as an example. [Figure 6] This graph shows the relationship between strain rate and grain size, using Nb-added steel as an example. [Figure 7]It is a graph showing the relationship between temperature and the volume fraction of Nb-based precipitates. [Figure 8] It is a flowchart shown to explain the manufacturing method of a forged product by hot forging (direct forging quenching). [Figure 9] This shows a comparison between the direct forging quenching according to this embodiment and the conventional post-forging quenching. In the figure (a), it is a graph showing the relationship between time and temperature by direct forging quenching, and in the figure (b), it is a graph showing the relationship between time and temperature by post-forging quenching. [Figure 10] It is a diagram showing a comparison of chemical components for a prototype example (test piece) of a forged product by direct forging quenching according to this embodiment. [Figure 11] It is a graph illustrating the temperature change pattern of the processing formaster test. [Figure 12] It is a graph showing the crystal grain size according to the heating temperature of a test piece by the processing formaster test. [Figure 13] It is a list showing examples of the test content for a test piece. [Figure 14] It is an optical microscope photograph showing an example of the test result for a test piece. [Figure 15] It is a magnified photograph showing the distribution of precipitates. In the figure (a), it is the case of base steel, in the figure (b), it is the case of Nb0.05, in the figure (c), it is the case of Nb0.10, and in the figure (d), it is the case of increased Nb0.05+Ti. [Figure 16] It shows the relationship between the strain rate and strain with respect to the grain size in base steel. In the figure (a), it is a graph when the heating temperature is 1170°C, and in the figure (b), it is a graph when the heating temperature is 1250°C.

Mode for Carrying Out the Invention

[0016] Hereinafter, the hot forging steel according to the embodiment of the present invention and its manufacturing method will be described in detail with reference to the drawings.

[0017] Embodiment Figure 1 shows a comparison of the chemical composition of forged products (hot forging steel) produced by direct forging and quenching according to this embodiment.

[0018] In this embodiment, as shown in Figure 1, for example, hot compression tests were performed using a processing formaster testing machine on four types of steel with different content of each component.

[0019] Here, steel type (1) is a boron steel that serves as the base steel, and contains the following components: 0.38 wt% C (carbon), 0.27 wt% Si (silicon), 1.03 wt% Mn (manganese), 0.98 wt% Cr (chromium), 0.033 wt% Al (aluminum), 0.03 wt% Ti (titanium), 0.002 wt% B (boron), and 0.006 wt% N (nitrogen).

[0020] Steel type (2) is an Nb-added steel (base + Nb0.02 steel) in which 0.02 wt% Nb (niobium) that completely dissolves when heated to a temperature of 1250°C is added, with the boron steel of steel type (1) as the base steel (containing 0.38 wt% C, 0.28 wt% Si, 1.03 wt% Mn, 0.99 wt% Cr, 0.032 wt% Al, 0.02 wt% Ti, 0.002 wt% B, and 0.010 wt% N as its respective components).

[0021] Steel type (3) is an Nb-added steel (base + Nb0.05 steel) which uses the boron steel of steel type (1) as the base steel (containing 0.38 wt% C, 0.28 wt% Si, 1.03 wt% Mn, 1.00 wt% Cr, 0.045 wt% Al, 0.02 wt% Ti, 0.002 wt% B, and 0.010 wt% N as its respective components), with an additional 0.05 wt% Nb added.

[0022] Steel type (4) is an Nb-added steel (base + Nb0.10 steel) in which 0.10 wt% Nb is added, which remains partially undissolved when heated to a temperature of 1250°C, to a base steel of the boron steel of steel type (1) (containing 0.38 wt% C, 0.27 wt% Si, 1.03 wt% Mn, 1.00 wt% Cr, 0.028 wt% Al, 0.03 wt% Ti, 0.002 wt% B, and 0.008 wt% N as components).

[0023] In other words, by utilizing Nb-based precipitates that are stable within the hot forging temperature range, that is, by performing hot forging on boron steel to which a predetermined amount of Nb has been added, strain-induced precipitation is generated, and fine Nb-based carbide particles are dispersed at high density.

[0024] When using carbonitrides of Ti in boron steel as carbide precipitation sites, for example, there is a concern that adding large amounts of Nb will increase the solution treatment temperature and cause coarsening due to compounding with other carbonitrides during precipitation, thereby reducing the pinning strength. Therefore, it is necessary to optimize the amount of Nb added (a predetermined amount) according to the conditions during hot forging.

[0025] As will be described in more detail later, in this embodiment, the growth of minute crystal grains generated by dynamic recrystallization is suppressed by adding an optimal amount of Nb to the base steel according to the conditions during hot forging. In this way, it is possible to obtain forged products with fine crystal grains (high grain size) even when performing direct forging and quenching.

[0026] The crystal grain size (D) after dynamic recrystallization can be determined, for example, by equation (1) in [Equation 1] below.

[0027]

number

[0028] Figure 2 is a TEM (transmission electron microscope) image showing a magnified view of the distribution of Nb-based precipitated carbide particles when the amount of Nb added is 0.05 (Nb0.05).

[0029] Nb added to the base steel forms coarse crystal precipitates during casting, but dissolves upon heating and re-precipitations as fine Nb carbide particles during cooling before forging and due to strain induction during forging, as shown in Figure 2, for example. Since these Nb precipitates are stable within the hot forging temperature range, for example around 1100°C, it is expected that grain growth will be suppressed due to the grain growth pinning effect at high temperatures.

[0030] Figure 3 shows the relationship between temperature and the volume fraction of (Nb,Ti)C under equilibrium conditions, illustrating the amount of precipitation during cooling from the heating temperature (1250°C) to the forging temperature (1155°C).

[0031] In Figure 3, the horizontal axis represents the heating temperature (°C), and the vertical axis represents the volume fraction (%) of (Nb,Ti)C, comparing the results when the amount of Nb added is 0.02 (0.02Nb), 0.05 (0.05Nb), and 0.10 (0.10Nb).

[0032] As is clear from Figure 3, the temperature during solid solution and precipitation changes depending on the amount of Nb added. Therefore, it is necessary to optimize the amount of Nb added to the base steel according to the other components and the conditions during hot working. This makes it possible to suppress the growth of minute crystal grains generated by dynamic recrystallization.

[0033] Figure 4 shows the relationship between the amount of Nb added and the grain size. Here, the horizontal axis represents the amount of Nb added (wt%) and the vertical axis represents the grain size (μm). The figures show the results for heating temperatures of 1250°C (marked with ● in the figure) and 1170°C (marked with ◇ in the figure), with Nb added amounts of 0.00 (base steel), 0.02, 0.05, and 0.10.

[0034] As is clear from Figure 4, it was found that grain refinement is possible when Nb is added to the base steel, both at heating temperatures of 1250°C and 1170°C.

[0035] In other words, in Nb-added steel, the grain size decreases as the amount of Nb added increases, and it was confirmed that the effect of grain refinement improves with increasing Nb content.

[0036] Figure 5 shows the relationship between strain rate and grain size, using base steel (steel type (1)) as an example. Here, the horizontal axis represents true strain rate (s). -1 The vertical axis represents the grain size (μm), and the forging temperature is set to 1235°C and 1155°C, which are assumed to be the temperatures used when actually manufacturing forged products, and the strain rate is set to 0.01 to 10 s. -1 This shows the case where the range is specified. Note that the strain rate can be calculated from the compression rate.

[0037] As is clear from Figure 5, obtaining fine crystal grains (e.g., 60 μm or less) by dynamic recrystallization takes approximately 0.1 seconds at a forging temperature of 1235°C. -1 While the above strain rates are required, at a forging temperature of 1155°C it is approximately 0.1 seconds. -1 It was confirmed that this is possible even at the following strain rates.

[0038] Figure 6 shows the results of a hot compression test using a processing formaster testing machine, with the horizontal axis representing the true strain rate (s). -1 This diagram shows the relationship between strain rate and grain size, with the vertical axis representing grain size (μm), using Nb-added steel (steel types (2) to (4)) as an example.

[0039] As is clear from Figure 6, in all types of steel, the grain size decreases as the strain rate increases, and it was confirmed that the grain can be further refined in Nb-added steel compared to base steel.

[0040] In other words, in Nb-added steel, the dynamic recrystallization structure, which becomes finer by increasing the strain rate, can be pinned down by Nb-based fine precipitated carbide particles, thus enabling the creation of a steel material that can be further refined.

[0041] However, as shown in Figure 5, in forging at high temperatures, the true strain rate is 0.1 (s) -1Below this point, grain coarsening was observed.

[0042] Figure 7 shows the relationship between temperature and the volume fraction of the Nb-based precipitate (Nb,Ti)C under equilibrium conditions. Here, the horizontal axis represents temperature and the vertical axis represents the volume fraction of the Nb-based carbide, and the results calculated using Thermo-Calc (integrated thermodynamic calculation software) are shown as an example.

[0043] Figure 7 also shows that the solid solution and precipitation temperatures of Nb-based precipitates differ depending on the amount of Nb added.

[0044] Next, the method for manufacturing forged products (steel materials) by direct forging and quenching will be explained with reference to the flowchart shown in Figure 8. Figure 9 shows a comparison between direct forging and quenching according to this embodiment (Figure (a)) and conventional post-forging quenching (Figure (b)).

[0045] In this embodiment, as shown in Figure 9(a), for example, by adding an optimal amount of Nb to a boron steel base to produce Nb-added steel, and then performing appropriate hot forging on this steel, it is possible to manufacture a steel material that can obtain a high grain size while suppressing grain coarsening.

[0046] In other words, in hot forging, the Nb-added steel is first heated to a predetermined heating temperature (for example, 1250°C) (step S01 in Figure 8).

[0047] Next, after a predetermined time has elapsed and the heating process in step S01 is completed, a forging process is performed on the Nb-added steel at a predetermined forging temperature (for example, 1155°C) (step S02 in Figure 8).

[0048] Next, after a predetermined time has elapsed and the forging process in step S02 is completed, the residual heat is used to perform a quenching process on the Nb-added steel (step S03 in Figure 8).

[0049] Thus, by properly performing a series of direct forging and quenching processes on Nb-added steel, it becomes possible to eliminate processes such as the normalizing process shown in Figure 9(b), enabling the production of forged products in a shorter time.

[0050] Moreover, it not only reduces costs and carbon emissions, but also enables the manufacture of forged products with desired material properties, such as a predetermined impact value.

[0051] As described above, according to this embodiment, grain coarsening can be suppressed even in direct forging and quenching without the normalizing process, making it possible to obtain a high grain size.

[0052] In other words, a predetermined amount of Nb is added to a pre-existing boron steel that serves as the base steel to optimize it, and during manufacturing, this Nb-added steel, which has this optimal amount of Nb added, is subjected to appropriate direct forging and quenching.

[0053] This allows the pinning force of the Nb-based precipitated carbide particles to be exerted, making it possible to suppress the growth of minute crystal grains generated by dynamic recrystallization.

[0054] Therefore, even in direct forging and quenching without the normalizing process, it is possible to obtain forged products with fine crystal grains while reducing costs and carbon emissions.

[0055] Furthermore, the base steel (boron steel) of steel type (1) may also contain approximately 0.05 wt% or less of Mo (molybdenum).

[0056] Furthermore, the C content should be within the range of 0.35 (min) to 0.39 (max) wt%, the Si content should be within the range of 0.15 to 0.35 wt%, the Mn content should be within the range of 0.85 to 1.05 wt%, the Cr content should be within the range of 0.90 to 1.00 wt%, the Al content should be within the range of 0.015 to 0.045 wt%, the Ti content should be within the range of 0.015 to 0.040 wt%, the B content should be within the range of 0.0005 to 0.0030 wt%, and the N content should be ~0.0070 wt% or less.

[0057] Prototype example Figure 10 shows a comparison of the chemical composition of a prototype forged product produced by direct forging and quenching according to this embodiment, as a prototype example.

[0058] In this prototype example, steel type (1) is a boron steel that serves as the base steel, and contains the following components: 0.38 wt% C, 0.27 wt% Si, 1.03 wt% Mn, 0.017 wt% P (phosphorus), 0.021 wt% S (sulfur), 0.98 wt% Cr, 0.033 wt% Al, 0.033 wt% Ti, 0.0019 wt% B, and 0.0055 wt% N.

[0059] Steel type (2) is an Nb-added steel (base + Nb 0.02 steel) in which 0.023 wt% Nb is added, which completely dissolves when heated to a temperature of 1250°C, to the boron steel of steel type (1) as the base steel (containing 0.38 wt% C, 0.28 wt% Si, 1.03 wt% Mn, 0.013 wt% P, 0.019 wt% S, 0.99 wt% Cr, 0.032 wt% Al, 0.022 wt% Ti, 0.0023 wt% B, and 0.0100 wt% N as its respective components).

[0060] When this Nb-added steel is used as a prototype, for example, as shown in Figure 3, all of the Nb dissolves at a heating temperature of 1250°C, making it possible to use the existing Ti-based carbonitride particles in the steel and the Nb-based carbide particles that re-deposit at low forging temperatures for pinning.

[0061] Steel type (3) is an Nb-added steel (base + Nb 0.05 steel) with the boron steel of steel type (1) as the base steel (containing 0.38 wt% C, 0.28 wt% Si, 1.03 wt% Mn, 0.014 wt% P, 0.021 wt% S, 1.00 wt% Cr, 0.045 wt% Al, 0.024 wt% Ti, 0.0024 wt% B, and 0.0095 wt% N as its respective components), to which 0.053 wt% Nb is further added.

[0062] When this Nb-added steel is used as a prototype, for example, as shown in Figure 3, at a heating temperature of 1250°C, not all of the Nb dissolves, and pinning occurs due to existing Ti-based carbonitride particles in the steel, undissolved Nb-based carbide particles, and finely dispersed Nb-based carbide particles due to reprecipitation.

[0063] Steel type (4) is an Nb-added steel (base + Nb 0.10 steel) which is made by using the boron steel of steel type (1) as the base steel (containing 0.38 wt% C, 0.27 wt% Si, 1.03 wt% Mn, 0.014 wt% P, 0.021 wt% S, 1.00 wt% Cr, 0.028 wt% Al, 0.025 wt% Ti, 0.0024 wt% B, and 0.0082 wt% N as its respective components), and further adding 0.101 wt% Nb, which remains partially undissolved during forging and quenching.

[0064] When this Nb-added steel is used as a prototype, for example, as shown in Figure 3, a large amount of Nb remains undissolved at a heating temperature of 1250°C. Pinning occurs due to the undissolved Nb-based carbide particles remaining in the steel, existing Ti-based carbonitride particles, and finely dispersed Nb-based carbide particles due to reprecipitation.

[0065] Steel grade (5) is an Nb-added steel (base + Nb 0.05, Ti-added steel) with the boron steel of steel grade (1) as the base steel (containing 0.37 wt% C, 0.27 wt% Si, 1.03 wt% Mn, 0.013 wt% P, 0.021 wt% S, 1.00 wt% Cr, 0.034 wt% Al, 0.033 wt% Ti, 0.0024 wt% B, and 0.0086 wt% N as its respective components), to which 0.049 wt% Nb is further added.

[0066] When this Nb-added steel is used as a prototype, an increase in the amount of precipitate is expected due to the synergistic effect between Nb-based precipitates and Ti-based precipitates.

[0067] Figure 11 illustrates a processing formaster pattern, with time on the horizontal axis and temperature on the vertical axis, showing the temperature change of the test material during the processing formaster test.

[0068] In Figure 11, the temperature change pattern shown by the solid line represents an example where the heating temperature is 1250°C, while the temperature change pattern shown by the dashed line represents an example where the heating temperature is 1170°C.

[0069] Figure 12 shows the grain size (AGS) of prototypes (steel grades (1) to (4)) at different heating temperatures, obtained from the Processing Formaster test.

[0070] As is clear from Figure 12, it was confirmed that a higher grain size (fine grains) can be obtained when the heating temperature is lower.

[0071] Figure 13 is a table showing the contents (conditions) of the processing form master test for the prototype.

[0072] For prototypes, tests are conducted under 1 to 10 different conditions, as shown in Figure 13, for example.

[0073] Figure 14 shows a comparison of the test results (former austenite grain structure) of the prototype. Here, the results of the processing formmaster test under conditions 1 to 10 shown in Figure 13 correspond to the cross-section of the prototype.

[0074] In other words, the processing formaster test (hot compression test) on the prototype is performed using a processing formaster testing machine, which is not shown in the diagram here.

[0075] More specifically, for example, as a prototype, Nb-added steel containing a predetermined component was cast and forged, and a cylindrical test piece with a diameter of 8 mm and a length of 12 mm was cut from it and heated to a temperature of 1170°C to 1250°C. Then, 6.2s -1 After applying compressive deformation at a true strain rate such that the processing rate was 75%, the specimen was rapidly cooled. The resulting specimen was then cut longitudinally and used to observe the cross-section of the prototype.

[0076] Figure 15 is a TEM image showing a magnified view of the precipitate distribution. Figure (a) shows the case of base steel, Figure (b) shows the case of Nb-added steel (Nb 0.05), Figure (c) shows the case of Nb-added steel (Nb 0.10), and Figure (d) shows the case of Nb-added steel with the same Ti content as base steel (Nb 0.05 + increased Ti).

[0077] As shown in Figure 15(a), in the base steel, relatively large Ti(C,N) precipitates are prominent as Ti-based precipitates.

[0078] As shown in Figure 15(b), in the Nb-added steel (Nb 0.05), fine NbC precipitates, and core-shell formation is observed where some Ti(C,N) atoms are encased.

[0079] As shown in Figure 15(c), in the Nb-added steel (Nb0.10), fine NbC precipitates, and core-shell formation is observed where some Ti(C,N) is encased.

[0080] As shown in Figure 15(d), in Nb-added steel (Nb 0.05 + increased Ti), although NbC formed a core-shell structure, the coarsening of Ti(C,N) resulted in a decrease in precipitates, and no synergistic effect was observed.

[0081] These findings confirm that the addition of Nb resulted in the dispersion of fine precipitated particles.

[0082] Figure 16 shows a comparative relationship between strain rate and strain with respect to grain size in the base steel. Figure (a) is an example where the heating temperature is a low 1170°C (compression temperature of 1155°C), and Figure (b) is an example where the heating temperature is a high 1250°C (compression temperature of 1235°C).

[0083] As is clear from Figures 16(a) and (b), grain size is dominated by strain rate and temperature (Zener-Hollomon factor), and decreases with increasing strain rate and decreasing compression temperature. This is because a larger Zener-Hollomon factor results in more recrystallization nuclei during dynamic recrystallization, leading to a smaller average grain size.

[0084] Therefore, in the case of Nb-added steel, which is made by adding Nb to a base steel containing C, Si, Mn, P, S, Cr, Al, Ti, B, and N as components, as in the prototype, grain growth after dynamic recrystallization can be suppressed by optimizing the amount of Nb added. As a result, grain coarsening can be suppressed even in direct forging and quenching without the normalizing process, and a high grain size (fine grains) can be obtained. [Industrial applicability]

[0085] This invention can be used in various applications, including the manufacture of undercarriage components for construction machinery by construction machinery manufacturers, as well as the manufacture of automotive parts such as hubs by automobile manufacturers.

Claims

1. A steel for hot forging in which grain growth is suppressed by directly forging and quenching an Nb-added steel, which is a base steel to which a predetermined amount of Nb has been added.

2. The base steel is boron steel, The hot forging steel according to claim 1, wherein each chemical component contains at least 0.35 to 0.39 wt% of C, 0.15 to 0.35 wt% of Si, 0.85 to 1.05 wt% of Mn, 0.90 to 1.00 wt% of Cr, 0.015 to 0.045 wt% of Al, 0.015 to 0.040 wt% of Ti, 0.0005 to 0.0030 wt% of B, and 0.0070 wt% or less of N.

3. The boron steel further contains 0.05 wt% or less of Mo, as described in claim 2, for hot forging.

4. The hot forging steel according to claim 2, wherein the boron steel further contains 0.017 wt% of P and 0.021 wt% of S.

5. The hot forging steel according to claim 1, wherein 0.02 to 0.10 wt% of Nb is added as the predetermined amount of Nb.

6. A heating step is performed on an Nb-added steel to which 0.02 to 0.10 wt% Nb is added, wherein the Nb-added steel is to be made by adding 0.02 to 0.10 wt% Nb to a base steel containing at least 0.35 to 0.39 wt% C, 0.15 to 0.35 wt% Si, 0.85 to 1.05 wt% Mn, 0.90 to 1.00 wt% Cr, 0.015 to 0.045 wt% Al, 0.015 to 0.040 wt% Ti, 0.0005 to 0.0030 wt% B, and 0.0070 wt% or less N, respectively. A forging process is performed in which the Nb-added steel is subjected to a hot forging treatment, following the heating process. A quenching process is performed on the Nb-added steel using the residual heat from the hot forging process, A method for manufacturing hot forging steel equipped with [a specific feature / feature].

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